This invention is concerned with a synthetic functional fluid base stock and more particularly with certain polyol ester base stocks. Polyol esters are formed from the reaction of neopentyl type polyhydric alcohols and monocarboxylic acid compositions. These can be exemplified by trimethylol propane triheptanoate, a widely used commercial ester prepared from trimethylol propane and n-heptanoic acid. These esters are useful as base stock components in various types of lubricants and especially in crankcase oil formulations designed for gasoline, turbine and diesel engines.
Lubricants comprised of synthetic base stocks have a number of performance advantages over traditional lubricants containing naturally occurring mineral oil base stocks. Synthetic lubricants have demonstrated greater thermal stability, broader viscosity profiles, lower volatility and better frictional properties than mineral oils.
Automobile engines have become smaller to reduce vehicle weight and thereby save fuel. However, the thermal stress on the engine lubricant has increased concomitantly. The replacement of natural oils with more thermally stable synthetic base stocks, such as polyol esters can extend the lubricant's useful life significantly, e.g. from 7,000 miles to over 25,000 miles. Improved thermal-oxidative stability of the polyol ester contributes to the extended life of the lubricant.
A further advantage of synthetic lubricants, such as polyol esters, is that they have very broad viscosity profiles. As a result, lubricants can be formulated with synthetic base stocks that exhibit fluidity at much lower temperatures than presently possible with mineral oils and still provide adequate viscosity at hot operating temperatures. Improved cold weather starting and improved fuel economy result from good low temperature properties.
Synthetic base stocks also have improved frictional properties over mineral oils which is attributed to their chemical composition. Both the improved frictional properties and low temperatures fluidity are factors which contribute to improved fuel economy.
The lower volatility of synthetic lubricant base stocks also reduces the oil consumption rate.
While synthetic lubricant base stocks in general, and polyol esters in particular, have considerable performance advantages over mineral oil, there are problem areas. Generally, polyol esters and conventional engine oil additives are non-miscible with each other. Polyol esters also have deleterious effects on engine seals, excessive swelling being a big problem.
Seal "swell" is defined as the amount in percent that the volume of an elastomer engine seal expands upon contact with, and exposure to the lubricant environment under engine operating conditions. Insufficient or excessive swell causes the seals to lose their ability to retain and confine the engine fluids. Leakage occurs which can cause a high amount of oil consumption.
A controlled seal swell, sufficient to prevent lubricant leakage is therefore one of the most important properties of a crankcase lubricant.
The polyol esters of this invention are unique in that they overcome the additive miscibility and seal swelling problems normally associated with other polyol esters. Furthermore, it is desirable when formulating synthetic crankcase lubricants to use a polyol ester with both a high 210.degree. F. viscosity and a low pour point. The high 210.degree. F. viscosity will minimize the need for adding polymeric viscosity index improvers which thicken the formulation but also contribute to instability and engine deposits. Low pour point esters impart good low temperature fluidity to the lubricant composition. The polyol esters of this invention are unique in that they exhibit higher 210.degree. F. viscosities and lower pour points than comparable esters without iso-palmitate.
British Patent Specification No. 1,444,826 discloses the usefulness of iso-palmitate polyol esters in hydraulic fluids. This patent confines itself to full iso-palmitate esters.